Catalysts for polymerization of conjugated dienes such as a 1,3-butadiene and an isoprene have been conventionally proposed a lot and some of them have been industrialized. For example, a method of manufacturing conjugated diene polymers with a high cis-1,4 structure often uses a compound of titanium, cobalt, nickel or neodymium and an organoaluminum in combination.
The use of a group-3 element of the periodic table as a catalyst for polymerization of conjugated dienes has been well known, and various polymerization methods have been proposed until now. For example, JP-A6-228221 discloses a solid catalyst holding a support for (co)polymerization of conjugated dienes, which holds a compound of at least one of metals having atomic numbers 57-71, 92 on a support. An yttrium catalyst having an atomic number 39 is hardly described however.
JP-A 7-70143 discloses an organometallic complex including yttrium (Y), neodymium (Nd) or praseodymium (Pr) and a group-13 element. No example of polymerization of yttrium complexes is described however.
JP-A 7-268013 describes a catalytic system including neodymium (Nd), praseodymium (Pr), dysprosium (Dy), lanthanum (La), gadolinium (Ga) and yttrium (Y) in combination with an aluminum alkyl and a trialkyl derivative of boron. It exemplifies methods of polymerizing conjugated diene compounds, which are though limited in neodymium and praseodymium.
JP-A 8-325330, JP-A 9-151219, JP-A 10-60174, JP-A 11-217465 and JP-A 11-222536 exemplify yttrium as a metal to be turned into a catalyst for manufacture of a cis-1,4-polybutadiene but fail to provide any specified example using an yttrium catalyst.
JP-A 2003-226721 discloses a method of manufacturing a cis-1,4-polybutadiene in the presence of a catalyst, which is a compound of an element selected from the group consisting of scandium, yttrium, lanthanides and actinides. No specified example using an yttrium catalyst is shown, however, and exemplification of the method of polymerizing conjugated diene compounds is limited in neodymium and praseodymium.
A Polybutadiene has a bonded portion (1,4-structure) generated from polymerization at the 1,4-site and a bonded portion (1,2-structure) generated from polymerization at the 1,2-site, which coexist in a molecular chain as the so-called microstructure. The 1,4-structure is further classified into two: a cis structure and a trans structure. On the other hand, the 1,2-structure is structured to have a vinyl group in a side chain.
As known, depending on polymerization catalysts and polymerization conditions, polybutadienes different in the above microstructure are produced and employed in various uses in accordance with their properties.
For the purpose of improving the heat radiation and abrasion resistance of tires, blending a polybutadiene rubber (BR) in natural rubber and so forth is widely performed, and various BRs are proposed. For example, JP-A 7-118443 discloses a BR having a weight average molecular weight of 500,000-750,000, a molecular weight distribution of 1.5-3.0, and an inherent viscosity of 90 or more. JP-A2001-247721 discloses a BR having a cis content of 95% or more and a molecular weight distribution of 3.5-6.0.
In a rubber composition for golf balls, particularly, a high-cis polybutadiene with a relatively narrow molecular weight distribution and a high molecular linearity has a property excellent in abrasion resistance, resistance to heat radiation, and rebound resilience. As an index of linearity of high-cis polybutadienes with almost similar molecular weight distributions, Tcp/ML1+4 is used. Tcp indicates the degree of molecular entanglement in a thick solution. The lager the Tcp/ML1+4, the smaller the branch degree and the larger the linearity is.
Golf balls are classified into a thread wound type and a solid type. The solid center in the thread wound ball, as well as the solid ball, conventionally includes a rubber base material such as a polybutadiene, and a monomer having an unsaturated bond, such as an unsaturated metal carboxylate, compounded therein as a crosslinking coagent. A peroxide and a metal oxide are also compounded therein.
The polybutadiene used as the rubber base material of golf balls is generally required to have high rebound and excellent processability. A higher Mooney viscosity improves the rebound but worsens the processability while a wider molecular weight distribution improves the processability but lowers the rebound in an antinomy relation.
For the purpose of achieving the compatibility of processability and rebound, improvements in polybutadiene rubber have been tried and various proposals have been provided. For example, JP-A 63-275356 and JP-A 2-177973 disclose polybutadienes having a high Mooney viscosity and a wide molecular weight distribution and synthesized in the presence of a Ni-based catalyst. JP-A 6-80123 discloses a method that uses a polybutadiene having a low Mooney viscosity blended with a polybutadiene having a high Mooney viscosity.
JP-A 7-268132 discloses the use of a polybutadiene, having a cis content of 97% or more and modified with a tin compound, as a rubber base material for golf balls. This remains unchanged in the crosslink density, however, compared to the conventional high-cis polybutadiene, and accordingly an improvement is desired in durability.
The Inventor et al. disclose in JP-A 2001-40040 that a polybutadiene appropriately having the 1,2-content can be used in long-carry golf balls.